# Yeah, but WHY is c the speed limit of the universe? **URL:** \ **Category:** Factual Questions\ **Tags:** science-math\ **Created:** [October 15, 2024, 5:03pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837 "2024-10-15T17:03:26Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\ **Post date:** [October 15, 2024, 5:03pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/1 "2024-10-15T17:03:26Z")
I understand that as an object nears the speed of light its mass increases and approaches infinite mass so it’s near its maximum speed. But my question is why the speed of light? What is so magical about it that it presents such a limit to the entire universe. Is this a matter of “that’s just the way the universe works”? That, I can accept. Is that also the way mathematics works? ---
**Author:** ![Sage\_Rat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sage_rat/32/399_2.png) [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)\ **Post date:** [October 15, 2024, 5:09pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/2 "2024-10-15T17:09:04Z")
I’m sure that a physicist will jump in briefly to say more but my understanding is that it’s not particular to light. Under relativity, you’re dealing with the relationship between two things (mass and energy, time and distance), where the more you grab of one the less you have of the other. More mass is less energy, more energy is less mass, etc. If you run the math for the speed of a hypothetical object of zero mass, you get the same speed that we observe light to be moving. This is also the speed of other massless particles. It could more properly - again, as I understand it - be termed the maximum speed of massless particles. Light is just the most prominent example. ---
**Author:** ![Buck\_Godot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/buck_godot/32/6573_2.png) [@Buck\_Godot](https://boards.straightdope.com/u/Buck_Godot)\ **Post date:** [October 15, 2024, 6:14pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/3 "2024-10-15T18:14:03Z")
Yes basically, there is a constant c, that can be expressed as a velocity. And according to the relativity, for particles with mass, their energy increases asymptotically as their speed approaches this value c. Thus in order to reach c it would require infinite energy, so no massive particle can move at c or faster Meanwhile a zero mass particle moving less than c, has zero energy and zero momentum which means it has no way to affect other matter and so for all intents and purposes it doesn’t exist. Photons are massless particles that have energy, therefor they have to move at this constant c. So we call c the speed of light. Meanwhile if you want to imagine tachyons that move faster than c, you find that according to their reference frame you can have events which reverse their order, so that effect happens before cause, which if they were able to transmit information would lead to all sorts of paradoxes. So if such particles “exist” they would not affect our universe and so for all intents and purposes might as well not exist. ---
**Author:** ![DorkVader](https://avatars.discourse-cdn.com/v4/letter/d/73ab20/32.png) [@DorkVader](https://boards.straightdope.com/u/DorkVader)\ **Post date:** [October 15, 2024, 6:15pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/4 "2024-10-15T18:15:58Z")
I would reframe your question like this, “why is c the speed limit of light and causality for all of the stuff in the universe?” I do that as a nitpicky thing, the universe (space specifically) separate from the stuff in it, is not limited to the speed of light. The best answer I can give is that because it is. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [October 15, 2024, 7:11pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/5 "2024-10-15T19:11:42Z")
Here are two earlier threads. There are more. > [@Why is light used as the unique descriptor for the universal speed limit?](https://boards.straightdope.com/t/why-is-light-used-as-the-unique-descriptor-for-the-universal-speed-limit/615445): > > As per the Wikipedia entry for “Speed of Light”: [Speed of light - Wikipedia](http://en.wikipedia.org/wiki/Speed_of_light) Apparently the top speed of light is a much more general concept than the name alludes to, because this same top speed also applies to phenomena such as information and gravity (theoretically). Why is light so fundamental out of all of these phenomena that it deserves a monopoly on the name for the universal speed limit? > [@Does the speed of light limit apply to the universe as a whole?](https://boards.straightdope.com/t/does-the-speed-of-light-limit-apply-to-the-universe-as-a-whole/662298): > > Years ago I read an article (I think it was in Popular Science) that a group of scientists had discovered two objects were further apart in light years than the age of the universe and the angle of their divergence could account for. Is this theoretically possible? Could the fabric of space actually be expanding faster than the speed of light? ---
**Author:** ![UncleMoose](https://avatars.discourse-cdn.com/v4/letter/u/e0b2c6/32.png) [@UncleMoose](https://boards.straightdope.com/u/UncleMoose)\ **Post date:** [October 15, 2024, 7:12pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/6 "2024-10-15T19:12:32Z")
The shortest explanation I’ve seen for the numbers (that fall out of Einstein’s equations) is that for a photon, the speed of light is where the time dilation is infinite (time slows down to zero) and the length contraction in the direction of motion shrinks to a zero size. So the actual speed isn’t random, it falls out of the general relativity equations. ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [October 15, 2024, 7:24pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/7 "2024-10-15T19:24:41Z")
I think @Chronos explained it in some thread somewhere that you should think of c as the same for everything moving through space-time, and with more useful (for this conversation) units, would have the value of 1. So, if you’re at rest, you’re moving through time at 1 second per second. As you speed up, your movement through time slows, until you reach c and you’re now moving through time at 0 seconds per second. c is the slope of that space-time movement. Anyway, something like that. Hopefully, he’ll swing by and correct all my mistakes. ---
**Author:** ![FlikTheBlue](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fliktheblue/32/10972_2.png) [@FlikTheBlue](https://boards.straightdope.com/u/FlikTheBlue)\ **Post date:** [October 15, 2024, 7:26pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/8 "2024-10-15T19:26:10Z")
I’ve also heard it described as “everything travels at the same velocity”. It’s just that things with mass, like us, have that velocity oriented in the time direction. Because we travel through space so slowly, we instead travel through time at almost 1 second per second. Massless particles don’t experience time and very light particles like neutrinos travel at a lot less than 1 second per second through the time vector of movement. ETA. Ninjad. > [@RitterSport](#): > > So, if you’re at rest, you’re moving through time at 1 second per second. As you speed up, your movement through time slows, until you reach c and you’re now moving through time at 0 seconds per second. c is the slope of that space-time movement. Sounds similar to what I heard. If this is the right explanation, the question then becomes why is the value of that slope c. I think the best answer we’ll ever come up with is “it just is”. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [October 15, 2024, 7:35pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/9 "2024-10-15T19:35:36Z")
My thought is that _c_ is the limiter based on the fabric of the universe i.e. the electric permittivity and magnetic permeability. You simply cannot have information move faster than electromagnetism would allow. I have been told I’m wrong and I probably am but honestly I have been very disappointed with discussions of the speed of light here. When I asked why the speed of light what it is, 90% of the responses were about units and how we measure things and if it were bleeps per blorps there would be different numbers. Completely missing what I was asking. I see some of this here which boils down to: That is what the equations say; If you reach a velocity of _c_ then that quantity becomes zero (or infinite). I don’t know if that is what you are looking for but what I found that answered it to my satisfaction was that if information moved faster than causality (that’s what the _c_ represents) that certain paradoxes would occur. For me that is an explanation because it give the why of it beyond mathematical manipulation. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [October 15, 2024, 7:45pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/10 "2024-10-15T19:45:05Z")
The concept of a 4-dimensional spacetime was introduced to describe Lorentz-invariant physics, and of course this symmetry is experimentally tested, but if the OP’s question is _why_ the universe has such a law, I am not sure there is a succinct answer. You cannot merely say “that is what the equations say” because that is begging the question. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [October 15, 2024, 7:48pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/11 "2024-10-15T19:48:13Z")
> [@FlikTheBlue](#): > > I’ve also heard it described as “everything travels at the same velocity”. I’ve also heard that we shouldn’t think of it as just the speed of light in a vacuum, but (more generally) as the _speed of causality_. ---
**Author:** ![FlikTheBlue](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fliktheblue/32/10972_2.png) [@FlikTheBlue](https://boards.straightdope.com/u/FlikTheBlue)\ **Post date:** [October 15, 2024, 7:48pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/12 "2024-10-15T19:48:36Z")
> [@Saint\_Cad](#): > > I see some of this here which boils down to: That is what the equations say; If you reach a velocity of _c_ then that quantity becomes zero (or infinite). I don’t know if that is what you are looking for but what I found that answered it to my satisfaction was that if information moved faster than causality (that’s what the _c_ represents) that certain paradoxes would occur. For me that is an explanation because it give the why of it beyond mathematical manipulation. I think it’s because there’s two separate but related questions, and it needs to be clarified which one is being asked. 1. Why is there a speed limit to causality? It’s because, as you say, if there wasn’t such a limit, then the whole notion of casualty breaks down and we get paradoxes. 2. Why is the specific number of that speed limit c? I don’t know, but my guess is that it’s probably not possible to know why c is the particular number that it is. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [October 15, 2024, 7:55pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/13 "2024-10-15T19:55:16Z")
> [@FlikTheBlue](#): > > Why is the specific number of that speed limit c? That’s easy to answer: it’s just a meaningless random number. You can choose units so that _c_ = 1. ---
**Author:** ![FlikTheBlue](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fliktheblue/32/10972_2.png) [@FlikTheBlue](https://boards.straightdope.com/u/FlikTheBlue)\ **Post date:** [October 15, 2024, 8:01pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/14 "2024-10-15T20:01:42Z")
> [@DPRK](#): > > That’s easy to answer: it’s just a meaningless random number. You can choose units so that _c_ = 1. Sure. But it being meaningless and random isn’t really an answer. It’s just an explanation as to why the particular value that c happens to be can’t be determined through scientific inquiry. What that number is? Sure, we can answer that. Why that particular value? As you say, it’s most likely random, and so trying to answer why that particular number is unlikely to give a useful answer. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [October 15, 2024, 8:05pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/15 "2024-10-15T20:05:43Z")
Probably the only way to know why _C_ has its particular value, rather than say a million times faster or slower, would be to find a Theory of Everything in which ALL the universal constants of physics could be shown to be interrelated in a way which dictated their magnitudes. ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [October 15, 2024, 8:25pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/16 "2024-10-15T20:25:54Z")
Science doesn’t really answer “why” questions, but maybe this is a good way to think about it, hopefully not too wrong, similar to my explanation above: Time ticks forward at a certain rate (why? that’s the way it is), and bodies at rest tick forward through time at that rate. Using the right units, call that 1 time period/time period, maybe? Then, when you’re in motion, you’re still moving through time, and now also moving through space. Your speed through time/space is unchanged, still 1, but now you have a vector, with one component being your movement through 3-D space and one through time. The faster you go, the more your vector is composed of the 3-D space part, less at the time part, until you’re moving at 1, all through 3-D space, and time is stopped for you. c, in our arbitrary units, represents that movement through 4-D spacetime. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [October 15, 2024, 8:31pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/17 "2024-10-15T20:31:07Z")
> [@RitterSport](#): > > Science doesn’t really answer “why” questions, Science can answer lots of “why” questions _at the upper levels_. As you dig deeper into the “why” behind each feature of each answer, you slowly burrow down the ultimate base of How the Universe Works. You’re quite right that at that very base level, Science semi-fails and we’re left with the observation that “Our universe works this way and we’re fortunate enough to be here to witness it doing so.” With the anthropic side order of “If it did work differently we would not be there to see it. Some other intelligence might be and they’d be just as insistent that their observations are simply a necessity as we are about our observations being so.” ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [October 15, 2024, 8:36pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/18 "2024-10-15T20:36:26Z")
> [@Buck\_Godot](#): > > Meanwhile if you want to imagine tachyons that move faster than c, you find that according to their reference frame you can have events which reverse their order, so that effect happens before cause, which if they were able to transmit information would lead to all sorts of paradoxes. So if such particles “exist” they would not affect our universe and so for all intents and purposes might as well not exist. Just as a point of note, tachyons are a consequence of mathematical symmetry, and there is neither physical evidence for tachyons nor a theoretical need for tachyons in either the Standard Model or in quantum field theories in general. (There is the concept of a tachyonic field with imaginary mass such as the Higgs field, and it is a common mechanism in condensed matter physics but these fields cannot propagate information at speeds faster than _c_, and are really an indication that there is some deeper physics that is not understood.) There are interpretations of quantum mechanics, such as some flavors of the “transactional interpretation” (and something like a tachyon naturally falls out of any time-symmetric and retrocausal interpretations) but these interpretations produce intrinsically unfalsifiable claims and generally make my head hurt, and are largely discounted as useful interpretations outside of philosophical discussions. _c_ is an (apparently) fundamental and there is no particular reason that we know of that it has to be this particular value other than that it is consistent with observation and confirmed theory. However, if it were much slower, we would (likely) have a very empty visible universe as it would be outpaced by the expansion of spacetime, and if it were much faster we would have a vastly more energetic universe in which chemistry (at least, as we know it) would not be possible. There are also a vast number of other physical mechanisms that would be impacted that would make the universe not inhabitable for us, although this should not be taken as any evidence of an “anthropocentric universe”, since if the fundamental constants were not the way they are we just wouldn’t be here to notice. Stranger ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [October 15, 2024, 8:38pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/19 "2024-10-15T20:38:50Z")
> [@Stranger\_On\_A\_Train](#): > > this should not be taken as any evidence of an “anthropocentric universe” My favorite analogy of this is the water in a puddle thinking, why, this space just fits me perfectly! It must be designed for me. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [October 15, 2024, 8:57pm UTC](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837/20 "2024-10-15T20:57:30Z")
The difference between “anthropic” and “anthropocentric” is a lot more than just the letters “ocentr”. [Next page](https://boards.straightdope.com/t/yeah-but-why-is-c-the-speed-limit-of-the-universe/1008837.md?page=2)